There are many kinds (types) of zeolites called LTA, MFI, MOR, AFI, FER, FAU, and DDR classified by the crystal structures.
Of these, DDR (Deca-Dodecasil 3R) is a crystal having silica as the main component and having pores by a polyhedron containing an oxygen-containing eight-membered ring in the crystal structure. The pore size of the DDR-type zeolite is 4.4×3.6 angstrom, which is relatively small among pore sizes of various zeolites.
The DDR-type zeolite has an inherent adsorption property with respect to low molecular gas in addition to a small pore size. Therefore, the DDR-type zeolite is used as an adsorber for separating only a specific low molecular gas. For example, the DDR-type zeolite can be used as a molecular sieve membrane for low molecular gas.
As a process for producing a DDR-type zeolite, there is known a process of growing crystals of the DDR-type zeolite by using 1-adamantaneamine as a structure-defining agent, using colloidal silica, ethylenediamine, and water as the raw materials, and adding DDR-type zeolite seed crystals to the raw materials for hydrothermal synthesis. In this production process, there can be produced an all silica DDR-type zeolite not containing aluminum (Patent Documents 1 and 2).